The present invention relates to a plastic injection molding machine and, more particularly, a plastic injection molding machine having a plasticizing unit and an injection unit.
Plastic injection molding machines typically employ plasticizing units and injection units for injecting resin into a mold. Reciprocating screws are generally employed for plasticizing plastic resin and either directly injecting the melted resin into a clamped mold or transferring the melted resin to a shooting pot. Various drive means can be employed to both rotate the screw for plasticizing and translating the screw inside the barrel so that it acts as an injection piston to force melted resin in front of the screw into the mold or into a shooting pot.
The plasticizing unit conventionally includes a heated cylinder which receives a plasticizing screw which is capable of rotational and axial movement inside the heated cylinder. Pellets of plastic are introduced into the heated cylinder and melted therein. Rotation of the plasticizing screw forces the pellets along the length of the heated cylinder until they form a pool of melted resin in front of the screw tip. A check valve provided on the screw tip prevents backflow of the melted resin as the screw is axially advanced into the cylinder by the injection unit thereby causing the resin to be forced out of the nozzle in the end of the heated cylinder. The nozzle on the end of the heated cylinder is sealed against the sprue bush of a mold and the action of forcing the resin into the mold is called plastic injection. Typically during the operation of the plastic injection molding machine the plasticizing screw in the cylinder, which both rotates and slides, develops high injection pressures typically in the area of 30,000 psi.
There have been various designs for the plasticizing and injection units of plastic injection molding machines in order to enable the plasticizing screw to both rotate and axially move within the heated plasticizing cylinder. A typical device is described and illustrated in U.S. Pat. No. 4,731,005 to Hehl. In the arrangement of the Hehl patent as best shown in FIG. 3, the injection piston 58 of the injection unit has a motor 63 attached directly to it for rotating the plasticizing screw. The motor in turn drives a quill 64 which is attached to the plasticizing screw 14. As the motor 63 rotates, the plasticizing screw 14 is driven via the quill 64 through a series of fixed couplings. As the pool of melted resin builds up in the front of the plasticizing screw 14, the screw is pushed toward the right along with the injection piston 58 which slides inside injection cylinder 57. Thrust bearing 55 transmits this back force to the injection piston while permitting the screw 14 to rotate. When sufficient resin has been plasticized, motor 63 is stopped in order to stop rotation of the plasticizing screw 14 and high pressure fluid is delivered into the injection cylinder 57 and acts on the righthand side of the injection piston 58 which causes the injection piston 58, the quill 64 and the plasticizing screw 14 to be moved to the left so as to inject the plastic resin from the cylinder and into a mold. As can be seen from the foregoing discussion and particularly FIG. 3 of the Hehl patent, the motor 63 reciprocates back and forth with the screw 14 and the injection piston 58. As a result of the fact that the motor is fixed to the injection piston and reciprocated therewith, the weight and inertia of the motor increases the wear on the bearings and seals of the assembly which is highly disadvantageous. In addition, a complicated attachment is required from the power supply to the reciprocating motor 63.
U.S. Pat. No. 5,173,312 to Julinot teaches a non-rotating injection piston containing a rotating quill for connecting the rotary motion from the motor to the screw. This unit requires more expensive bearings and is slightly longer when assembled. Also, the lubrication of the bearing nearest the screw is not conveniently executed. Finally, the diameter of the assembly of the '312 patent is relatively large for a given injection piston area because the '312 injection piston is annular.
Accordingly, it is a principal object of the present invention to provide an improved injection molding machine and the operation thereof including a plasticizing screw, injection piston and quill.
It is a further object of the present invention to provide an improvement as aforesaid which is simple and effective in operation.
It is a still further object of the present invention to provide an improvement as aforesaid which has an improved size configuration and which conveniently executes operations.
It is an additional object of the present invention to provide an improvement as aforesaid which provides a cost saving operation.
Further objects and advantages of the present invention will appear hereinbelow.